This invention relates to automatic serial fuse blow circuits utilizing electrical fuses.
Memory devices, such as dynamic random access memory devices (DRAM) typically include fuse circuits. Fuses included in these fuse circuits may be employed to activate redundant elements which replace failing cells or components.
Fuses may be grouped into two classes, for example, laser fuses and electrical fuses. Laser fuses are blown by a chip-external laser beam which irradiates specific fuses to supply enough energy to blow the fuses. Electrical fuses are blown when a current through the fuses exceeds a threshold causing energy build-up to blow the fuse. Electrical fuses are often utilized for modern semiconductors. Electrical fuses allow for adjustments and repairs to be performed as late as in the packaged chip. Also, electrical fuses are likely to replace laser fuses since electrical fuses allow for more design flexibility, for example, fuse position within the chip, wiring above the fuse and wiring below the fuse.
Known electrical fuses can be a wire type or a semiconductor layer type. For the wire type, a relatively low-resistive element, such as a wire, increases its resistance permanently once a high current stresses this element. The stressed wire creates an opening similar to traditional laser-fuses. For the layer type fuse, a high-resistive layer between conductive layers permanently decreases its resistance from a high voltage or consecutive current, leading to a breakdown of this layer. For example, an isolator or dielectric material which starts to break through at a certain voltage level and stays permanently damaged forming a low resistive path. The first type of fuse will be referred to as type A; the second type of fuse will be referred to as type B.
One of the major differences between laser fuses and electrical fuses is the manner in which the fuses are addressed or accessed, so that the fuses can be blown. For laser fuses, the laser beam is simply pointed at the fuses which are to be blown, that is, addressing by location. Unlike laser fuses, electrical fuses are not blown using addressing by location, electrical fuses are blown by accessing pins of a chip.
It is desirable for electrical fuses to play an increasing role in higher density memory devices. Considering the large number of fuses typically found on DRAM circuits, if standard decoding schemes are applied, the selection of an electrical fuse that should be blown either requires a large amount of decoder circuitry or many address wires on a chip.
It is often considered a disadvantage of electrical fuses that the actual time, from the moment that the fuse starts to blow until the fuse is completely blown, is not instantaneous. The moment when the fuse starts to blow, is the moment when the current through the fuse starts to decrease for type A and starts to increase for type B.
To get a reliable and permanent change of resistance, especially for the fuse type B, it is desirable to be able to control the after-blow current and the time period during which this current is applied. If the fuses are blown by applying a certain voltage or current for a certain predetermined blow timetappl, then this predetermined blow time needs to be chosen at or above the upper end of the statistical time distribution to achieve a high fuse blow success rate. However, the after-blow time is then unknown. The range of the after-blow time will vary across the whole blow time distribution (tmin thru tmax) and will be in between times, tappl-tmax and tappl-tmin. Using the predetermined blow time tappl for each fuse can result in very long blow times. For example, if this concept is used to repair modern DRAMs using redundant elements with thousands of fuses per chip, then the blow time will be very long since the blow time will be related to the number of fuses to be blown. Therefore, attempts are made to blow the fuses in parallel to shorten the total blow time. Blowing the fuses in parallel creates additional problems and obstacles for both types of fuses.
For type A fuses, it is theoretically possible to blow fuses in parallel assuming enough current can be provided to assure the desired stress conditions. Wiring limitations will create additional resistance. Therefore, the stress conditions will be different from the first-blowing fuse, which uses the lowest voltage and individual current due to the highest overall current, to the last blowing fuse, which uses the highest voltage and current due to lowest overall current. The lower individual voltage and current at the beginning of the parallel blow attempt can have an influence on the dynamic process of the fuse-blow and on the predetermined blow time tappl needed.
For type B fuse parallel blow, the voltage will be highest for the first-blowing fuse having minimal current as long as all the fuses are high-resistive. The voltage will be lowest for the last-blowing fuse. A harder-to-blow fuse will use a reduced voltage after the initial high voltage which blew its neighboring fuses. This harder-to-blow fuse, which was not destroyed at the initial high voltage level, will certainly demonstrate an even greater reluctance to being blown at a lower voltage level. Maximum blow time tmax will increase and it will be even harder to specify a predetermined blow time tappl.
A need therefore exists fogs a fuse blow control circuit which makes parallel fuse blow unnecessary, therefore, avoiding the problems mentioned above.
According to a preferred embodiment of the present invention, a fuse blow control circuit is provided wherein the blow time is self-regulating, every fuse takes its own tome needed to blow plus an additional adjustable or predefined after-blow time. The blow process and success can be monitored and parts of the circuit with faulty fuses can be sorted out immediately.
The fuse blow process can be an automatic serial process. For example, control signals are not required once the process is started. After expiration of a predetermined time period, fuse blow success can be monitored by a tester using a monitoring device. The predetermined time period can be calculated based on a fuse string, on a chip by chip basis, or the predetermined time period can simply be chosen so that it fits a worst-case scenario test case. If fuse blow success is detected by a tester then no further testing of the fuses is necessary. However, the monitoring of fuse blow success can be repeated if there are additional fuses to be blown. Furthermore, if it is desirable to increase fuse reliability, or to blow fuses again in case the fuses have failed after stress, the tester can adjust the predetermined time period and can continue to monitor fuse blow success.
An embodiment of the invention includes a method for controlling fuse blow. The method can have the following steps: feeding a Blow_Go signal into a fuse latch then blowing a fuse. After the fuse is blown, automatically gating through the Blow_Go signal to another fuse latch. The steps of gating through the Blow_Go signal and blowing fuses are repeated until successfully blowing all of the to-be-blown fuses.
Another embodiment of the present invention includes feeding a Select signal into a blow control circuit. Once the Select signal is fed into the blow control circuit, the Select signal is evaluated to determine if a Blow_Go signal should blow the fuse. If the fuse is not to be blown, then the Blow_Go signal is automatically gated through to the next fuse latch. If the fuse is to be blown, the blow control circuit blows the fuse, shuts off the device that is connected to the fuse and automatically gates through the Blow_Go signal to the next fuse latch. The automatic gate through of the Blow_Go signal and evaluation of the Select signal keep repeating until successfully blowing all of the to-be-blown fuses.
In a further embodiment of the present invention, a delay timer waits for a fuse to be blown. After the delay timer detects that a fuse is blown, the delay timer is started. The delay timer will control the after-blow time of the fuse before shutting off the device that is connected to the fuse. Once the delay timer is started, further fuse blow processing waits until the delay timer has expired. After the delay timer has expired, a Blow_Stop signal is issued that causes a blow control circuit to shut off the device that is connected to the fuse.
Another embodiment of the present invention includes a circuit configuration that processes a digital input signal. The circuit configuration includes a blow control circuit that has an input side for receiving a Blow_Go signal. The circuit configuration also has a blow transistor for blowing at least one fuse. The blow transistor will blow a fuse if a Blow_signal is received and the fuse string information indicates that the fuse should be blown. Additionally, the circuit configuration has an output side for supplying a Blow_Go signal to a next fuse latch for possible further fuse blow processing after fuse processing within the current fuse latch is completed.
A further embodiment of the circuit configuration can include an input side for receiving a Select signal. The blow control circuit can have a signal evaluator circuit for evaluating the Select signal. Upon evaluation of the Select signal by the signal evaluator circuit, a determination is made as to whether or not the fuse should be blown. If the signal evaluator circuit determines that a fuse is to be blown based on the Select signal, the Blow_Go signal will activate the blow transistor that will blow at least one of the fuses.
An embodiment of the circuit configuration can also include a delay timer circuit. The delay timer circuit controls the after-blow time of the fuse. The delay timer circuit waits for a fuse to be blown. Once a fuse is blown the delay timer circuit is activated. Further fuse blow processing waits until expiration of a time period controlled by the delay timer circuit. Once the delay timer has expired, the blow device that is connected to the blown fuse is shut off. Then, the output side of the circuit configuration will supply a Blow_Go signal to a next fuse latch. The circuit configuration will repeat this process until all of the to-be-blown fuses are successfully blown.